In patients with acute or chronic kidney failure, impaired renal function must be compensated by alternative processes. Such alternative processes are hemodialysis and peritoneal dialysis. In continuous ambulatory peritoneal dialysis (CAPD), as it is called, the peritoneal cavity of patients with kidney disease is filled with a fresh peritoneal dialysis solution several times a day. In this type of dialysis, detoxification and dehydration occurs via the peritoneal membrane lining the entire abdominal cavity. In the exchange of substances the peritoneum forms a semipermeable membrane which the dissolved substances pass through by diffusion. Within two to three hours, as a result of diffusion there is a concentration increase, in the freshly filled peritoneal dialysis solution, of the substances usually excreted with the urine.
At the same time, the fluid is removed by ultrafiltration in accordance with the osmotic balance. The peritoneal dialysis solution remains in the abdominal cavity for 4 to 8 hours and is then drained to the outside through a catheter. As a rule, this procedure occurs four times a day, i.e., except for the inflow and outflow times, the peritoneal cavity is constantly filled with solution.
In addition to electrolytes and the osmotically active substance or substances, CAPD solutions contain a buffer substance, which regulates the acid-base metabolism of the patient, and, in particular, is intended to prevent or correct acidosis. Heretofore, sodium lactate has essentially been used to effect buffering. But studies of the biocompatibility of CAPD solutions have shown that solutions containing lactate have a negative effect on the vital function of human cells (cf., e.g., Bronswijk, Verbrugh, Bos, Heezius, Oe, van der Meulen, Verhoef: Cytotoxic effects of commercial continuous ambulatory peritoneal dialysis (CAPD) fluids, Peritoneal Dialysis International 9:197-202, 1989; or Witowski, Topley, Jorres: Effect of lactate-buffered peritoneal dialysis fluids on human peritoneal mesothelial cell interleukin-6 and prostaglandin synthesis, Kidney International 46:282-293, 1994). Bicarbonate-containing CAPD solutions with a physiological pH have proven to be better tolerated (cf., e.g., Schambye, Pederson, Christensen, Berthelsen, Wang: The cytotoxicity of CAPD solutions with different bicarbonate/lactate ratios, Peritoneal Dialysis International 13 (Suppl. 2):116-118, 1993).
From EP 0 339 549, a CAPD solution is known which is produced by mixing two compartments, one of which contains bicarbonate ions. The resulting solution mix has a physiological pH in the 7.2 to 7.4 range and contains 20 to 40 mmol/L bicarbonate ions. Production of the solution containing bicarbonate is difficult because after dissolution of the salt, carbon dioxide always forms in balance according to the following formula: EQU (1)2HCO.sub.3 -.revreaction.Co.sub.2 +CO.sub.3.sup.2- +H.sub.2 O
Carbon dioxide can leave the solution during preparation, stirring, and storage, which leads to an increase in the pH. In order to maintain the pH within the desired pH range, carbon dioxide or some other acid must be regularly added to the solution before filling. In the latter case, however, the bicarbonate content drops, which presents a problem with regard to the declared content and, hence, with regard to the usability of the solution for CAPD.
CAPD solutions are mostly filled into flexible plastic containers which are then subjected to sterilization with superheated steam. Although sheets with a barrier effect against CO.sub.2 are available, this process unavoidably results in the loss of CO.sub.2 and an increase in the pH. A further difficulty with the CAPD solution is that when the pH is too high, carbonate develops, which forms a relatively insoluble precipitate, for example in the form of a calcium carbonate deposit, with electrolytes present in the solution, especially calcium ions. Adjustment of the pH is therefore of decisive importance for the stability of the solution. Known from EP 564 672 A1 is a peritoneal dialysis solution of physiological composition with regard to the pH, in which the stability of the pH is improved. This peritoneal dialysis solution is obtained directly before use from two single solutions, with the first single solution containing an osmotically active substance and the second single solution containing bicarbonate ions. The first single solution contains anions of mono- and/or dicarboxylic acid and has a pH of 4.5 to 5.8, and the second single solution contains an amino-acid component or a peptide component and has a pH of 7.2 to 10.0. The solution ready for use contains 23 to 26 mmol/L of bicarbonate ions. With this previously known peritoneal dialysis solution it is still necessary to mix together the solution finally used from the separately prepared single solutions and then to use it within a short time.
The considerations referred to above with regard to peritoneal dialysis can generally be carried over to an infusion or dialysis solution containing bicarbonate.
Common to all processes is that, within the context of sterilization with superheated steam or heat sterilization to be undertaken, CO.sub.2 escapes and the pH therefore increases.